1. Field of the Invention
The present invention related generally to a charge transfer device, and is directed more particularly to a charge transfer device suitable for use with a solid state image sensor.
2. Description of the Prior Art
In a solid state image sensor there is provided a charge carrier storage portion which may store a charge carrier in response to the received light amount during the light receiving period. In such the solid state image sensor, when an excess charge carrier, which exceeds an amount of charge capable of being stored in the storage portion, is locally or partially generated, the excess carrier will flow to other storage portions or other picture elements to cause blooming.
In order to avoid the generation of the blooming, there has been proposed various methods for the formation of a solid state image sensor which will remove the excess carriers.
For example, in a solid state image sensor of the frame transfer system which uses a surface channel charge transfer device, an excess charge is injected in the form of minority carrier to a substrate (bulk) region.
Now, turning to FIG. 1, a prior art surface channel charge transfer device of the two-phase type will be now described. In the example of FIG. 1, there is provided a semiconductor substrate, for example, silicon substrate 1 of the P-type. On one surface of the silicon substrate 1 there is provided an insulating layer 2 on which a plurality of electrodes 3A and 3B are formed. Every other electrode are made as sets, and clock voltages .phi.1 and .phi.2 are applied to two sets of electrodes 3A and 3B, respectively. In this case, the thickness of the insulating layer 2 beneath the respective electrodes 3A and 3B, 3 is so selected that it is different at the front and rear stages with respect to the charge transfer direction to produce an asymmetrical surface potential. The charge is transferred by the two-phase clocks in a predetermined direction or in the direction of arrow a in FIG. 1. During the light receiving period, one of the electrode sets, for example, electrode set 3A is supplied with, for example, a predetermined negative voltage and hence the substrate surface beneath the electrode set 3A is caused to be in an accumulation state for the majority carrier, while the other electrode set 3B is supplied with, for example, a positive predetermined voltage and hence the substrate surface beneath the electrode set 3B is put in a depletion state or an inversion state. At this time, the potential becomes as indicated by a solid line b in FIG. 1, and the potential in the accumulation state is a little higher (shallower) than the substrate potential indicated by a broken line c in FIG. 1. Accordingly, under such a state when light is irradiated on the substrate 1 from, for example, its rear surface and charge carriers are generated therein in response to the received light amount, the minority carriers are stored in the potential well beneath the electrode set 3B i.e. storage portion for the minority carrier with the amount being in response to the received light. In this case, if a charge carrier whose amount is greater than the charge supplied by the storage portion is produced, an excess charge does not flow on the accumulation surface but is injected into the bulk substrate whose state is lower than the accumulation surface in the form of minority carriers. As set forth, the excess carrier flows to the bulk substrate to avoid having excess charge flows to the other storage portions i.e. to other picture elements, or the excess charge is transferred together with the charges of the other picture element. In this case, however, the excess charge is injected into the bulk substrate as minority carriers and diffused in the bulk substrate equally in all directions, so that part of the carrier arrives at the storage portions of other picture elements and is caught thereby. Thus, the above prior art method can not remove the blooming phenomenon completely.
Also, in a solid state image sensor of the inter-line transfer system, as shown in FIG. 2, there is provided a semiconductor substrate, for example, silicon substrate 11 of the P-type. On one surface of the silicon substrate 11 there is provided an insulating layer 12 on which a sensor electrode 13 made of, for example, a transparent electrode material is formed to receive light. Thus, there is formed a sensor portion for storing the charges proportioned to the amount of the received light i.e. light receiving and storage portion. In this case, adjacent the sensor portion there is provided an overflow drain region 14 of the opposite conductivity type from that of the substrate 11, for example, N-type region which faces the surface of the substrate 11. Further, a transfer gate electrode 15 is provided between a shift register portion (not shown) and the sensor portion.
With the above prior art solid state image sensor of FIG. 2, during the light receiving state, the sensor electrode 13 is supplied with, for example, a positive predetermined voltage. In this case, for example, the thickness of the insulating layer 12 is varied so that there is provided a portion in which a potential well indicated by a solid line part d1 of a solid line d showing a surface potential distribution of the substrate 11 in FIG. 2 is produced i.e. the storage portion for the minority carriers and also a portion which forms a potential barrier indicated by a solid line part d2 of the line d between the storage portion and the overflow drain region 14. Thus, excess charges in the storage portion which are more than can be held by the storage portion which is determined by the barrier d2 are absorbed into the overflow drain region 14 as indicated by an arrow e in FIG. 2. However, the provision of the overflow drain region 14 as set forth above results in a manufacturing process which is complicated and also, due to the provision of the region 14 in the substrate 11 which has a different conductivity type than the region 11 and also due to the portion for the electrode required to supply a necessary voltage to the region 14, an excess area in the substrate 11 occurs the elements cannot be increased to handle higher charge density.